ARM: 7925/1: mm: keep track of last ASID allocation to improve bitmap searching

Since we only clear entries in the ASID bitmap on a rollover event, the
bitmap tends to consist of a block of consecutive set bits followed by
a block of consecutive clear bits. The exception to this rule is for
ASIDs which have been carried over from a previous generation, but
these are bound by the number of CPUs.

This patch optimises our bitmap searching strategy, so that we search
from the last successful allocation, rather than search from index 1
each time we allocate a new ASID.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
ARM: 7926/1: mm: flesh out and fix the comments in the ASID allocator

The ASID allocator has to deal with some pretty horrible behaviours by
the CPU, so expand on some of the comments in there so I remember why
we can never allocate ASID zero to a userspace task.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
ARM: mm: try to re-use old ASID assignments following a rollover

Rather than unconditionally allocating a fresh ASID to an mm from an
older generation, attempt to re-use the old assignment where possible.

This can bring performance benefits on systems where the ASID is used to
tag things other than the TLB (e.g. branch prediction resources).

Signed-off-by: Will Deacon <will.deacon@arm.com>

https://patches.linaro.org/39845/